"Eat fat, get fat" has been the conventional wisdom guiding American diets for the past two decades.

Yet more and more research suggests this kind of thinking is dangerously misguided.

Instead of finding evidence that low-fat eating plans offer health benefits, dozens of studies have suggested instead that these kinds of diets are unhelpful or potentially harmful. And new research points to another food category that could be the real problem: simple carbohydrates.

A large new analysis published August 29 in the journal The Lancet compared more than 135,000 people on low-fat and low-carb diets across 18 countries. The researchers found that low-fat diets were more likely to be linked with death from all causes, and found a higher likelihood of heart attacks and heart disease as well. People on low-carb diets, on the other hand, had significantly lower risk of both of these outcomes.

"Global dietary guidelines should be reconsidered in light of these findings," the researchers wrote in the paper.

The body needs some fat to function, so people on diets that lack this ingredient won't feel full longer term. Plus, fat helps you absorb vitamins and minerals, and is essential for blood clotting and muscle movement. It's also needed to build cell membranes (the shells that house each of your cells) and the protective shields around your nerves.

Part of the problem with low-fat diets is the processed, "low-fat" foods that are often advertised as healthy. When food-makers remove the fat from a product, they also remove most of its flavor, so often wind up adding other "filler" ingredients like sugar to compensate.

Because foods high in simple carbohydrates like white flour and sugar lack protein and fiber, they only satiate you briefly, leaving you hangry later. This is why they're often called "empty calories."

Not all fats are created equal

For that study, researchers tested what happened when thousands of people swapped out 5% of their calories from saturated fat (the type most often found in meat and dairy products) with calories from mono- and polyunsaturated fats (the kind found in olive oil, fish and nuts). They found that doing so was linked with numerous benefits, including a reduction in the risk of death and conditions like heart disease and several neurodegenerative diseases.

"Not all fats are created equal," Harvard nutrition professor Dr. Frank B. Hu, a lead author on the study, told The New York Times. "We should eat more good ones from fish and avocados, instead of animal fats," he said.